The f-Duct was invented by the McLaren F1 Team in 2010. It is a absolutely brilliant system to be faster (~0.2s in Monza on the start/finish-straight for example) on the straight lines.
The f-Duct got his name, because it was a duct, mounted on the nose of the McLaren MP4-25 right on the letter "f" of the sponsor logo of vodafone.
The duct was an air intake, which led air along the drivers cockpit towards the rear wing. And there was a hole in the drivers cockpit in this airflow system.
If the driver covered this hole with his hands (or later ellbows on other teams cars, when they adapted this system) the main airflow from the airbox (air-inlet above the drivers helmet) would be guided by the fluidic switch (where the airway from the f duct and the airbox encounter) through the upper pipework towards the rear wing. There the airflow would be then fed through additional slots in the rear wing itself causing the airflow around the wing to break down ("stall"). So it sort of cancelled out the airflow on the rear wing itself.
In this way the downforce on the F1 car was reduced, which gave it more straight line speed. Up to -0.2s in Monza for example.
If the driver let the hole open, the air was just directed into the cockpit without any aerodynamic effekt. The airflow from the airbox just ran through the fluidic switch and was now guided out through the neutral engine cover outlet (lower pipework towards the back-end of the car). Here as well without any aerodynamic effects. So the outer airflow over the bodywork of the moving car leading to the rear wing and generating downforce, isn't disturbed in this scenario.
In this way the car had the usual downforce in the curves, where it needed it.
So McLaren was able to fit rear wings with more downforce in for example the Monza GP 2010. In that way they hadn't have to compromise on downforce in the turns and nevertheless got more straight line speed out of their car.
Moving aerodynamic parts were forbidden in these days, so they evaded this rule by just using the pilot as a aerodynamic part of the f-Duct system. Brilliant!